This invention relates generally to receivers for use in conjunction with a global positioning system (GPS) and, more particularly, to demodulation and baseband processing techniques used in GPS receivers. The selection of intermediate frequencies and local oscillator frequencies used in a receiver is known as the frequency plan. As will shortly be appreciated, the choice of these and other parameters in the receiver represents a difficult task if all of the desired requirements of the receiver are to be met. For a better understanding of the various problems relating to choice of a frequency plan, the fundamental operation of the GPS will first be described.
GPS, also called NAVSTAR, is a system for determining the position of a user on or near the earth, from signals received from multiple orbiting satellites. When the system is fully deployed, the orbits of the satellites will be arranged in multiple orbit planes, such that signals can be received from at least four satellites at any selected point on or near the earth.
The orbits of the orbiting spacecraft are determined with accuracy from fixed ground stations and are relayed back to the spacecraft. In navigation applications of GPS, the latitude, longitude and altitude of any point close to the earth can be calculated from the times of propagation of electromagnetic energy from four or more of the spacecraft to the point on or near the earth. In general, at least four satellite signals need to be received at a ground station in order to determine the complete position, since there are four unknown quantities. Three of the unknowns are the three-dimensional position coordinates, which may be conveniently expressed in terms of latitude, longitude and altitude, and the fourth unknown quantity is a time difference or offset between timing clocks on the satellites and a timing clock at the receiver. For normal operation of the system, the clocks used to regulate operation of transmitters on the satellites are effectively all synchronized, or at least the differences between them are known. However, the clock used to control operations at the receiver is typically out of synchronization with the satellite clocks to some degree, and this error cannot be eliminated without having the benefit of one more satellite measurement than there are position coordinates to be determined. Thus, for three-dimensional position determination, at least four satellite signals are needed.
The nature of the signals transmitted from GPS satellites is well known from the literature, and will be described in more detail in the description of the preferred embodiment of the invention. In brief, each satellite transmits two spread-spectrum signals in the L band, known as L1 and L2, with separate carrier frequencies. Two signals are needed to eliminate an error that arises due to the refraction of the transmitted signals by the ionosphere. The satellite signals are modulated by two pseudorandom codes, one referred to as the C/A (coarse/acquisition) code, and the other referred to as the P (precise) code, and by a slower-varying data signal defining the satellite orbits and other system information. A pseudorandom code sequence is a series of numbers that are random in the sense that each one bears no discernible relation to the ones that precede it, but are not truly random, because the sequence repeats itself cyclically.
When a binary pseudorandom code is used to modulate the phase of a carrier signal, the result is a signal having a spectral density that follows a [(sin x)/x].sup.2 distribution. This "spread spectrum" signal has the advantage of being more immune to jamming or interference than a narrowband signal. The spectrum of a signal modulated by a pseudorandom code has the useful property that, when the signal is properly correlated with a replica of the same pseudorandom code, the spread spectrum energy is mapped as a large narrow peak in the spectrum, but only if the two correlated signals are properly synchronized in time. This property can be used to identify and separate signals from multiple satellites, by correlating a received signal with multiple locally generated pseudorandom code sequences. Each GPS satellite uses unique P code and C/A code sequences, which are publicly known. Therefore, a particular satellite is identifiable by the correlation of a received signal with a locally generated code sequence corresponding to that satellite. Once a received signal is identified and decoded, the receiver can measure an apparent transmission time from the satellite, from which an apparent range, or pseudo-range, is computed. Signals transmitted from each satellite define the time and position of the satellite at certain signal epochs whose times of reception can be measured at the receiver. The transmit times are all measured with reference to a common time base referred to as GPS system time. Each receiver uses its own local time reference for recording the receive times of signals from the satellites. Thus each receiver has knowledge of the transmit times measured in GPS system time and the receive times measured in local time. If there is at least one more satellite signal than there are positional unknown quantities, the time differential between the local time and satellite time can be determined along with the positional unknown quantities. For example, four satellite signals are needed to find three positional unknowns and the time differential. From the pseudo-range data, the position of the receiver on or near the earth can be computed to a high degree of accuracy, depending on the accuracy of the orbit data.
Demodulation of a carrier signal is usually performed by a process of multiplication of the received signal with a locally generated oscillatory signal, referred to as the local oscillator or LO. When two signal are combined in this way, the process produces components with frequencies that are the sum and the difference of the received and local frequencies. The sum components are usually discarded by appropriate filtering. The frequency difference components contain the same information as the received signal, but down-converted to a lower carrier frequency, referred to as an intermediate frequency. There may be several such intermediate stages of demodulation before the baseband signals are obtained, to provide the previously modulated information without carrier components. In a modern GPS receiver, the baseband signal is converted to digital form by a sampling stage.
One of the difficulties of devising a frequency plan for demodulating the L1 and L2 suppressed carrier signals is that it would be desirable to use a common reference frequency and to use, as far as possible, the same local oscillator frequency for L1 and L2 demodulation, at each stage of demodulation. This would result in a practically identical phase relationship between L1 and L2 and practically identical phase errors introduced in the demodulation process. However, although some frequency plans use the same local oscillator frequency at the first stage, it is thought to be difficult, if not impossible, to use identical local oscillator frequencies at every stage and still satisfy other important requirements for GPS demodulation.
Another important requirement is that it is highly desirable that the sampling stage should yield a signal that still contains a sufficiently high intermediate frequency, above baseband, to carry the coded information. If this requirement is not met, the sampling stage has added complexities of multibit sampling or sampling of both in-phase (I) and quadrature (Q) components of the signal. In addition, not addressing these added complexities in the receiver design will introduce unwanted signal attenuation and adversely affect the overall signal-to-noise ratio of the receiver.
Another general requirement is that the frequency plan should facilitate baseband processing of the signals in digital form. As will shortly be appreciated from the following summary, the present invention meets these and other additional requirements.